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Temporal Trends and Spatial Variation Characteristics of Hazardous Air Pollutant Emission Inventory from Municipal Solid Waste Incineration in China Hezhong Tian,* Jiajia Gao, Long Lu, Dan Zhao, Ke Cheng, and Peipei Qiu State Key Joint Laboratory of Environmental Simulation & Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China S Supporting Information *

ABSTRACT: A multiple-year emission inventory of hazardous air pollutants (HAPs), including particulate matter (PM), SO2, NOx, CO, HCl, As, Cd, Cr, Hg, Ni, Pb, Sb, and polychlorinated dibenzop-dioxins and polychlorinated dibenzofurans (PCDD/Fs), discharged from municipal solid waste (MSW) incineration in China has been established for the period 2003−2010 by using the best available emission factors and annual activity data. Our results show that the total emissions have rapidly amounted to 28 471.1 t of NOx, 12 062.1 t of SO2, 6500.5 t of CO, 4654.6 t of PM, 3609.1 t of HCl, 69.5 t of Sb, 36.7 t of Hg, 9.4 t of Pb, 4.4 t of Cr, 2.8 t of Ni, 926.7 kg of Cd, 231.7 kg of As, and 23.6 g of PCDD/Fs as TEQ (toxic equivalent quantity) by the year 2010. The majority of HAP emissions are concentrated in the eastern central and southeastern areas of China where most MSW incineration plants are built and put into operation. Between 2003 and 2010, provinces always ranking in the top three with largest HAPs emissions are Zhejiang, Guangdong, and Jiangsu. To better understand the emissions of these HAPs and to adopt effective measures to prevent poisoning risks, more specific field-test data collection is necessary.

1. INTRODUCTION With the rapid growth of social economy and improvement of public living standards, increased amounts of municipal solid waste (MSW) are being produced in China, especially in the urban areas. Normally, there are three main MSW disposal methods: landfill, composting, and incineration. Therein, incineration is a significant component of MSW management program in which solid organic wastes are subjected to combustion so as to convert them into residues and gaseous products, which can generate energy and simultaneously reduce the amount of waste by up to 90% in volume and 70% in weight.1,2 In order to restrict MSW pollution and substantial land occupation, a target rate of MSW harmless treatment of more than 80% by the year 2015 has been promulgated in the Chinese national 12th five-year-plan (2011−2015). Waste sorting and increasing the proportion of wastes burned are first proposed. Therefore, MSW incineration is going to enter a period of rapid development, especially in those cities where the local economy is relatively more developed and landfill sites are difficult to find. However, incineration is also regarded as a controversial method of waste disposal, due to the greatest environmental and human health concerns including discharge of particulate matter (PM), gaseous pollutants, and dangerous substances such as polychlorinated dibenzo-p-dioxins and © 2012 American Chemical Society

polychlorinated dibenzofurans (PCDD/Fs) and toxic heavy metals. Though MSW could be greatly reduced during the incineration process, many kinds of hazardous air pollutants (HAPs) are concentrated into the incineration byproducts, that is, air pollution control residues (APCs), bottom ash, and exhaust gas via physicochemical process.2,3 PCDD/Fs, commonly known as dioxins and furans, are known to be generated from incomplete combustion. PM emitted from MSW incinerators includes two categories: first, ultrafine PM even under 0.1 μm with nuclei mode and growth by coagulation and condensation in sound combustion with high temperature; and second, soot-type coarse PM, formed mechanically, such as incomplete combustion products under poor combustion conditions.4 Normally, HAPs emitted from MSW incineration exhaust can be classified into four categories: (1) acid gaseous pollutants (NOx, SO2, HCl, etc.), (2) incomplete combustion pollutants (CO, PCDD/Fs, etc.), (3) particulate matter, and (4) toxic heavy metals (Sb, Hg, Pb, Cr, Ni, Cd, As, etc.). Received: Revised: Accepted: Published: 10364

June 11, 2012 July 25, 2012 August 24, 2012 August 24, 2012 dx.doi.org/10.1021/es302343s | Environ. Sci. Technol. 2012, 46, 10364−10371

Environmental Science & Technology

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From the standpoint of environmental and health effects resulting from combustion of wastes, it is of great significance to ascertain the quantities, concentrations, and chemical forms of HAPs that are emitted from the stack. In order to restrain increasing emissions and pollution of key HAPs from incineration process, the Ministry of Environmental Protection of China (MEP) has officially ratified An Emission Standard for Pollution Control on the Municipal Solid Waste Incineration (GB 18485−2001).5 However, these HAPs cannot be removed completely from flue gas even when some advanced purification devices are installed and put into operation, and part of them will be released into the surrounding atmosphere. Additionally, the emission ceiling in the present standard (GB 18485−2001) is less stringent compared with those of developed countries (for more details, see Table S1 in the Supporting Information), which, to a certain degree, suggests that China still has further space for improvement in MSW incineration emission control. During recent years, emissions of HAPs from MSW incineration have received wide attention throughout the world. Particulate matter, heavy metals, gaseous pollutants, and PCDD/Fs emitted from MSW incineration to the atmosphere in some developed countries or districts, as well as global emissions, have been calculated and reported.3,4,6−11 Yoo et al.4 investigated the emission characteristics of PM and heavy metals from small-size incinerators and boilers. Werther9 analyzed the gaseous emissions from waste combustion and highlighted some current developments in this field. Quina et al.11 estimated the amount of PCDD/PCDF released in Portugal in 2006. In contrast, the relevant studies of HAPs emissions from MSW incineration in Chinese mainland are relatively limited.2,12−16 Zhang et al.2 studied the transfer behavior of heavy metals during MSW incineration. Streets et al.12 and Wu et al.13 suggested that MSW combustion as a nonnegligible anthropogenic source plays an important role in Hg emission to the atmosphere. Up to now, comprehensive and detailed investigations on the atmospheric emissions of HAPs during MSW incineration process in China have been quite limited. A complete and comprehensive emission inventory of key HAPs from MSW incineration with highly resolved temporal and spatial information is urgently needed in order to combat the increasing stress on urban and regional air pollution and poisoning risks. In this study, a multiple-year comprehensive emission inventory of key HAPs from MSW incineration processes for the period 2003−2010 in China is presented. Temporal variation trends and spatial distribution characteristics by province are analyzed in detail.

E T (t ) =

∑ ∑ Ai ,j(t )CEFi ,j(t ) i

=

j

∑ ∑ Ai ,j(t )Ci ,jR i ,j[1 − PPM(i ,j)][1 − PFGD(i ,j)] i

j

(1)

where E is the emission of HAPs; A is the annual activity level, representing the volume of MSW burned; CEF is called the comprehensive emission factor of HAPs for MSW incineration; C is the average content of some elements in MSW incineration consumption in one province; R is the fraction of air pollutants in flue gas released from MSW incineration facility; PPM and PFGD are the fraction of air pollutants removed by the installed PM collectors and flue gas purification (FGP) devices, respectively; j is the incinerator type (grate firing incinerator, GFI, or fluidized bed incinerator, FBI); i is the province; and t is the calendar year. Additionally, the uncertainties in this inventory are quantitatively assessed with Monte Carlo simulation. For parameters with adequate domestic measurement data, a probability distribution is fitted by use of Crystal Ball, while for parameters with limited observation data, probability distributions are assumed on the basis of the authors’ judgment or referred to relevant experts. Finally, all of the input parameters of activity levels and emission factors, with corresponding statistical distributions, are placed in a Monte Carlo framework, and 4000 simulations for each forecast variable are performed to analyze the uncertainties as well as which parameter significantly contributes to the uncertainties. 2.1. Municipal Solid Waste Incineration. Municipal solid waste (MSW)more commonly known as trash or garbage consists of everyday items including paper, plastics, textiles, food wastes, yard wastes, and other organic materials, as well as inorganic materials such as glass, metals, dirt, and miscellaneous other components. With the rapid urbanization process in China, especially in the eastern and coastal provinces with dense population and lack of adequate sites for landfill, incineration is playing a more and more important role in MSW management, mainly because it is an alternative waste treatment technology that can achieve substantial volume reduction, thorough stabilization, and good sanitation, as well as useful energy generation and recirculation. Since the first MSW incinerator was established in Shenzhen City of Guangdong province in 1988, more and more incinerator plants have been established and there have been 101 MSW incinerators with the normal rated treatment capacity of 84 940 t/day by the year 2010, reflecting a growth of 5.7 times compared with the year 2003.17 According to the statistical report issued by MEP, the volume of MSW incineration has amounted to 23.2 million tons by the end of 2010, accounting for 19% of the total MSW harmless treatment quantity. The rising tendency of MSW incineration capacity for the period of 2003−2010 in China is summarized in Figure 1. As can be seen, the MSW disposal capacity and the incineration capacity have been increasing annually, and the annual average growth rates during the period 2003−2010 are about 8% and 28%, respectively. The proportion of MSW incineration capacity is growing fast, from 6.8% in 2003 to 22% in 2010. The volume of MSW incineration at provincial level is compiled from China Statistical Yearbooks on Environment from 2004 to 2011.17 We distribute the total volume of MSW incineration in each province onto each plant based on the proportion of activity levels (MSW treatment capacity) of

2. METHODOLOGIES, DATA SOURCES, AND KEY ASSUMPTIONS In this study, emissions of key HAPs are calculated by combining the provincial-level statistical data on MSW incineration consumption and the best available specified HAP emission factors, which were subgrouped by taking accounting of different patterns of combustion facilities and the equipped PM collection and flue gas purification devices.17 The algorithm of a bottom-up emission inventory for MSW incineration can be expressed by the following equation:16,18 10365

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2.2. Incinerator Patterns and Flue Gas Purification System. Presently, two types of incineration technologies are mainly applied in China: grated firing incinerators, with a moving grate as combustion bed, and fluidized-bed incinerators, where the wastes are combusted when violently mixed and agitated with hot sand by air supplied from below. In addition, a few rotary kilns exist, but they are used only for the incineration of MSW mixed with special industrial or medical wastes. Grated firing incinerators (GFI) are widely used in the eastern coastal areas, especially in the large-scale capital and deputy provincial cities of China, taking a share of over 58% by the end of 2010; fluidized bed incinerator (FBI) technology, mainly distributed in the eastern medium-scale prefecture-level cities and the central and west areas of China, takes a relatively small proportion, due to necessary waste pretreatment, which is regarded as a relatively more expensive and complicated technology.14,19 For flue gas cleaning in MSW incineration plants of China, both types of incineration systems are equipped with semidry scrubber with slaked lime slurry injection to remove acid gases, activated carbon injection to absorb PCDD/Fs, followed by fabric filters to collect APCs including fly ash, reacted and unreacted lime, and activated carbon. The remaining flue gas will be emitted into the atmosphere through the stack. Normally, semidry scrubber with fabric filter (SD/FF) system is required for complying with GB 18485−2001 standard.5 The designed average removal efficiency for some plants is reported at about 99.5% for PM, 79% for SO2, 92.7% for HCl, and 95% for PCDD/Fs,20−24 and the average removal efficiency for heavy metals can achieve 99.9% for As, 99.7% for Cd, 99.7% for Cr, 99.3% for Ni, and 99.9% for Pb with SD/FF system.20,22,23,25,26 However, SD/FF system shows relatively low removal efficiency for Hg (about 51.1%) due to its high evaporation/condensation/adsorption process.20,22 In general, gas emissions are related more to the flue gas scrubbing system than to the incineration technology, but since both types of incineration employ the same gas purification system configuration, the differences in HAP emissions are primarily related to the adopted incineration technology. In fact, the differences are distinct. For example, NOx and PCDD/ F emissions from GFI are higher than those from FBI, while

Figure 1. Historical trend of MSW incineration capacity in China, 2003−2010.

individual plant, and we take into consideration the specific combustion and control technology configuration for each plant. Taiwan province, Hong Kong, and Macau Special Administrative Region are not considered tentatively. Also, Xizang Autonomous Region, the Inner Mongolia Autonomous Region, Jiangxi, Hubei, Hunan, Guizhou, Gansu, Qinghai, Ningxia, and Xinjiang Autonomous Region are not considered because they have no MSW incineration plants up to now. With regard to their spatial distribution, by the end of 2010, more than 2/3 of incineration plants were concentrated in the eastern and southern provinces of China. Zhejiang, Guangdong, and Jiangsu rank as the top three provinces accounting for 57% of the total amount of incineration plants in 2010 in China (Figure 2). For comparison, the provincial distribution of MSW disposal capacity and the MSW incineration capacity in 2010 are also given in Figure 2. The average MSW incineration capacity and the proportion of MSW incineration in total MSW harmless treatment capacity in the provinces with MSW incineration plants in Chinese mainland are 4045 t/day and 22%, respectively. Provinces with intensive economic activities and dense population such as Zhejiang, Fujian, Jiangsu, and Guangdong possess a relatively high proportion of MSW incineration, reaching 51%, 44%, 40%, and 35%, respectively. The proportion of MSW incineration in Heilongjiang (5%), Shaanxi (5%), and Hubei (8%) is much lower than in other provinces due to fewer incineration plants and lower incineration capacity in these provinces.

Figure 2. Regional distribution of MSW incineration plants and MSW incineration capacity, 2010. 10366

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CO and dust (PM10) data are lower for GFI.14 Figure 3 illustrates the composition of the two types of incinerator

Table 1. Average Comprehensive Emission Factors of Hazardous Air Pollutants from Municipal Solid Waste Incineration HAPs

Figure 3. Provincial composition of incinerator patterns in China, 2010.

technologies adopted in each province. Since the release rates of HAPs vary for different types of incinerator, we can anticipate that the average emission factors of HAPs for each province will vary with the changed proportion of incinerator types. 2.3. Comprehensive Emission Factors of Hazardous Air Pollutants in Flue Gas. The incinerator types for MSW combustion, the installed flue gas purification facilities, and the removal efficiencies of air pollutants through SD/FF control devices have been described previously. Thus, here we mainly focus on determination of the average CEF for different categories of HAPs in flue gas. Hasselriis and Licata6 calculated the emission factors of heavy metals from MSW incineration in modern waste-to-energy facilities in 1996, and their results were found within the limits of U.S. EPA (1993).27 Pacyna and Pacyna7 summarized the assumed average emission factors of trace metals from waste disposal according to some available data from a few countries in western Europe and the United States. Meanwhile, European Environment Agency (EEA) issued an air pollutant emission inventory guidebook, which has been updated in 2009.28 Emission factors of some air pollutants emitted from MSW incineration process were suggested in this guidebook. Because of the obvious difference of physical and chemical composition of wastes between China and developed countries, the best solution for assessing HAP emissions from MSW incineration is to measure the actual discharge rate, particularly the final stack discharge. Indeed, there have been some actual field tests conducted on Chinese MSW incinerators, which can be used to estimate the average emission factors of some air pollutants.14,20−26,29−31 Chen and Christensen14 calculated emission coefficients of PM10, NOx, SO2, CO, HCl, and dioxins for the two types of incinerators (GFI and FBI). Zhong26 investigated the emission concentration of PM, NOx, SO2, CO, HCl, Pb, Cd, and Hg from a MSW incineration plant. Here, we assume that about 5500 m3 of flue gas is emitted after burning 1 t of waste.20−22,25 Average emission factors of these air pollutants are obtained by multiplying the concentration of air pollutants by flue gas volume. Notably, we find that all of the tested MSW incineration plants are installed with the SD/FF purifying system to abate HAP discharge. Thus, we have compiled the averaged CEF of different HAPs on accounting of these domestic field tests results, which are summarized and illustrated in Table 1.

GFI, SD/FF, avg value (g/t)

PM NOx

236.7 1416.2

SO2

542.3

HCl

200.0

CO

159.7

Sb

3.0

Hg Pb Cr Ni Cd

1.4 0.4 0.19 0.12 0.04

As PCDD/ Fs

0.01 1.3 × 10−6

a

refs 21, 22, 24 14, 25, 26, 29, 30 14, 21, 22, 24−26 14, 20−22, 24, 26, 30 14, 25, 26, 29, 30 7, 16, 32

a

14, 20, 23, 25, 28 28 14, 23, 30 28 14, 30,

22, 30 26, 30

FBI, SD/FF, avg value (g/t)

refs

136.9 842.9

30 14, 30

484.5

14, 30

89.0

14, 30

481.8

14, 30

3.0

7, 16, 32 14, 30 14 28 28 14, 30

25, 26,

1.8 0.42 0.19 0.12 0.04

31

0.01 6.4 × 10−7

a

28 14, 30, 31

Given as grams TEQ (toxic equivalent quantity)/ton.

Presently, 64% of the GFI facilities in China are imported from developed countries. The majority of these imported technologies and facilities come from Europe, for example, Germany, where the most commonly installed flue gas purification system in MSW incineration plants is also the SD/FF system. Therefore, the average emission factors of HAPs in flue gas we used in this study are determined by combining domestic field tests with EEA results in order to get more reliable and complete estimations. Here the average emission factors of PM, SO2, NOx, CO, HCl, Pb, Cd, Hg, and PCDD/Fs are calculated on the basis of field tests results in domestic MSW incineration plants (see Table 1). PM, discussed in this study, refers to the total suspended particulates (TSP) according to GB 18485−2001 Standard. Owing to lack of available domestic field tests results, the average emission factors of As, Cr, and Ni are assumed as 0.01, 0.19, and 0.12 g/t according to EEA results, respectively.28 So far, there are few studies about Sb emission from MSW incineration. The emission factor of Sb is assumed as 3 g/t with reference to Pacyna and Pacyna7 and Tian et al.16,32 Also, here we assume that the average emission factors of HAPs did not change during the period 2003−2010 owing to available information limitation.

3. RESULTS AND DISCUSSION 3.1. Temporal Trends of Hazardous Air Pollutant Emissions from Municipal Solid Waste Incineration in China. According to the annual volume of MSW burned and the assumed average emission factors for different HAPs, the temporal trends of national total emissions of HAPs from MSW incineration in China during the period 2003−2010 are calculated, as illustrated in Table 2. As can be seen, the national total emissions of HAPs have increased rapidly, and the national total emissions are estimated at 4654.6 t of PM, 28 471.1 t of NOx, 12 062.1 t of SO2, 6500.5 t of CO, 3609.1 t of HCl, 69 501.9 kg of Sb, 36 686.4 kg of Hg, 9440.4 kg of Pb, 4401.8 kg of Cr, 2780.1 kg of Ni, 926.7 kg of Cd, 231.7 kg of 10367

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Table 2. Hazardous Air Pollutant Emissions from Municipal Solid Waste Incineration in China, 2003−2010 HAPs

2003

2004

2005

2006

PM (t)

622.6

826.5

1568.0

NOx (t) SO2 (t) CO (t) HCl (t)

4393.6 1694.3 1034.2 462.1

5089.6 2218.1 1302.0 639.5

Sb (kg) Hg (kg) Pb (kg) Cr (kg) Ni (kg) Cd (kg) As (kg)

9875.6 5353.6 1348.3 625.5 395.0 131.7 32.9

12 879.9 6922.6 1755.5 815.7 515.2 171.7 42.9

2331.7 Gaseous Pollutants 9533.4 14 193.2 4098.8 5942.6 2312.3 3080.9 1199.5 1813.0 Heavy Metals 23 713.5 34 128.0 12 637.2 17 873.0 3226.9 4628.7 1501.9 2161.4 948.5 1365.1 316.2 455.0 79.0 113.8

PCDD/Fs (g TEQ)

2.5

4.2

7.9

11.8

2007

2008

2009

2010

3029.9

3512.2

3988.0

4654.6

18 438.5 7546.4 3609.2 2347.6

21 398.8 8856.7 4412.1 2717.6

24 342.9 10 164.0 5220.1 3092.3

28 471.1 12 062.1 6500.5 3609.1

43 056.3 22 195.1 5822.4 2726.9 1722.3 574.1 143.5

50 696.1 26 340.3 6865.6 3210.8 2027.8 675.9 169.0

58 325.1 30 487.5 7907.7 3693.9 2333.0 777.7 194.4

69 501.9 36 686.4 9440.4 4401.8 2780.1 926.7 231.7

15.3

17.7

20.2

23.6

Table 3. Provincial Emissions of Hazardous Air Pollutants from Municipal Solid Waste Incineration in China in 2010 provinces

PM (t)

SO2 (t)

NOx (t)

CO (t)

HCl (t)

As (kg)

Cd (kg)

Cr (kg)

Hg (kg)

Ni (kg)

Pb (kg)

Sb (kg)

PCDD/Fs (g TEQ)

Beijing Tianjin Hebei Shanxi Jilin Heilongjiang Shanghai Jiangsu Zhejiang Anhui Fujian Shandong Henan Hubei Guangdong Guangxi Hainan Chongqing Sichuan Yunnan Shaanxi

211.1 138.1 111.7 71.6 68.4 22.7 256.1 965.1 852.0 68.4 336.6 180.0 90.1 28.6 850.7 14.4 11.3 88.6 179.9 106.4 3.0

482.7 315.9 303.4 253.5 242.0 80.3 585.7 2416.7 2266.9 242.0 769.8 637.3 318.8 91.1 1958.0 51.1 25.7 202.5 431.2 376.8 10.6

1300.3 851.0 688.0 440.6 420.7 139.5 1577.7 5944.1 5246.7 420.7 2073.5 1107.7 554.1 175.8 5045.6 88.8 69.4 545.6 1108.2 655.0 18.4

142.5 93.3 184.1 251.9 240.5 79.8 172.9 1127.2 1297.7 240.5 227.2 633.3 316.8 77.9 745.2 50.8 7.6 59.8 166.4 374.5 10.5

178.1 116.6 86.9 46.5 44.4 14.7 216.1 781.9 669.6 44.4 284.0 116.9 58.5 20.1 616.8 9.4 9.5 74.7 148.8 69.2 1.9

8.9 5.8 5.9 5.2 5.0 1.7 10.8 45.9 43.8 5.0 14.2 13.1 6.6 1.8 36.6 1.1 0.5 3.7 8.1 7.8 0.2

35.6 23.3 23.6 20.9 20.0 6.6 43.2 183.5 175.1 20.0 56.8 52.6 26.3 7.4 146.6 4.2 1.9 14.9 32.3 31.1 0.9

169.2 110.8 111.9 99.3 94.8 31.4 205.3 871.6 831.7 94.8 269.8 249.7 124.9 35.0 696.2 20.0 9.0 71.0 153.5 147.6 4.1

1273.6 833.6 957.0 961.6 918.2 304.5 1545.3 7060.5 7020.5 918.2 2030.9 2417.8 1209.4 324.8 5442.4 193.8 67.9 534.4 1202.5 1429.7 40.1

106.9 69.9 70.7 62.7 59.9 19.9 129.7 550.5 525.3 59.9 170.4 157.7 78.9 22.1 439.7 12.6 5.7 44.8 96.9 93.2 2.6

356.2 233.2 241.3 219.5 209.6 69.5 432.2 1859.3 1788.1 209.6 568.1 551.9 276.1 76.6 1475.6 44.2 19.0 149.5 325.4 326.3 9.2

2671.8 1748.7 1767.6 1567.8 1497.0 496.5 3241.8 13761.9 13132.8 1497.0 4260.6 3942.0 1971.9 552.0 10993.2 315.9 142.5 1121.1 2423.4 2331.0 65.4

1.1 0.7 0.6 0.3 0.3 0.1 1.4 5.1 4.4 0.3 1.8 0.8 0.4 0.1 4.0 0.1 0.1 0.5 1.0 0.5 0.01

kinds of pollutants constitute the majority of total HAP emissions. Sb and Hg are considered as the leading hazardous heavy metals of emissions. Although the absolute emissions of heavy metals are very low, the adverse impacts of these toxic heavy metals emitted to ambient air cannot be neglected. The shift and accumulation of heavy metals from the atmosphere to the surrounding water and soil environment should be highlighted, since they may transform to more toxic species and endanger the ecosystem and human health.33,34 In addition to the combustion technology and emission control technology employed in MSW incineration processes, waste composition also contributes to the variability of metals and PM emissions. Because of different life habits and seasonal and regional distribution, the components of wastes vary throughout the country. On the basis of historical data analysis, surveys in selected cities in China, and calculation of MSW, Du et al.35 indicate that the content of organic substances (food, textiles, wood, etc.) and reusable materials (paper, plastic,

As, and 23.6 g of PCDD/Fs as TEQ in 2010, with annual growth rates of 30−35% since 2003. With the increasing amounts of MSW produced during the rapid urbanization in China, land-use demand of city is increased; as a result, many cities have been surrounded by former deployed MSW landfills, and it is very difficult to find new suitable landfill sites. In addition, due to the localization of main incinerator equipment, the capital investment of incineration plant construction has been reduced gradually. Thus, more and more cities are considering establishing some incineration plants for MSW management and disposal, so the development of MSW incineration industries will be growing fast in the near future. Therefore, it is of great significance to be aware of the current status of key HAP emissions and implement adequate countermeasures to minimize their adverse effects on environment and public health. Particulate matter and gaseous pollutants are the main air pollutants in MSW incineration, and the emissions of these two 10368

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activity levels. To better understand the uncertainties in our inventory, Monte Carlo simulation was used to quantify the uncertainties of HAP emissions from MSW incineration. In this study, we assume normal distributions with coefficient of variation (CV, the standard deviation divided by the mean) of 10% for activity levels of MSW incineration and uniform distribution for comprehensive emission factors of HAPs.15,16,41 Table 4 illustrates the uncertainties (95% confidence interval around the arithmetic mean value) in Chinese atmospheric emissions of HAPs from MSW incineration in 2010.

rubber, etc.) in MSW in the south or in large cities (population greater than 500,000) is higher than in the north or in medium and small cities, while the inorganic substance content is reverse. Various waste compositions determine the different metal contents in wastes. According to the research of Li,36 colored plastic film, rubber, and mixed paper are the main sources of Pb. The major source of Hg is alkaline batteries: the concentration of Hg in alkaline batteries is 10 times or even hundreds of times greater than Hg in other waste (plastic, rubber, paper, textiles etc.), while there is little Hg content in food waste, grass, and leaf. Cr and Ni have similar sources and both of them reach the highest content in colored plastic film. Little Cr and Ni is found in other waste. Cd sources are primarily mixed rubber, fines (dirt), alkaline batteries, and lawn waste. Up to now, Chinese central and local authorities have actively developed and supported programs on collection and restriction of metal-containing wastes. Waste sorting is one of the effective ways, which can recycle waste paper, plastic, waste batteries, and so on. “Plastic bags ban” enacted on June 1, 2008, which may reduce or recycle metal-containing plastic packaging, and metals emission in these products will be decreased. Therefore, limiting and preventing hazardous metals from entering incinerators, via refuse sorting and recycling practices, is an effective way to control metal emissions in MSW incineration process. PCDD/Fs emissions in MSW incineration processes are lower than any other HAPs, while about 95% of atmospheric PCDD/F comes from the incomplete combustion of MSW incineration.37 Quina et al.11 and Zhang et al.38 indicate that the most significant route of PCDD/F emission is via residues, and about 90% of PCDD/Fs enter the environment through fly ash. Although the absolute emission of PCDD/Fs to air is not a very high value, it is of greatest significance, since these emissions may be dispersed through long distances, enter the food chains, and cause much public concern owing to their high toxicity and potential carcinogenic and mutagenic effects.38−40 Therefore, MSW incineration has become a very important source of PCDD/Fs emissions in China, which has aroused great concern from the public, especially those inhabitants in the surrounding areas of in-use or planned MSW incineration plants. For example, several new planned MSW incineration plants in Beijing and Guangzhou have encountered strong opposition from residents living nearby and have been re-sited or their construction has been stopped. 3.2. Distribution Characteristics of Hazardous Air Pollutant Emissions in 2010. Emissions of HAPs by province in China for the year 2010 are summarized in Table 3. As can be seen, HAP emissions from MSW incineration in China are distributed very unevenly. Jiangsu, Zhejiang, and Guangdong are the top three largest emitting provinces; furthermore, we find that Suzhou, Wuxi, and Nantong rank in the top three cities with largest HAPs emissions in Jiangsu province; Shaoxing, Cixi, and Ningbo are the top three cities with largest emissions in Zhejiang province; while HAPs emissions in Guangzhou, Shenzhen, and Dongguan of Guangdong province are highest. These cities are all located in the eastern and southern areas, especially some provincial capital and coastal cities. Heavy emissions are mainly driven by these areas with more developed local economy, denser population, and relatively large amounts of MSW output and incineration capacity. 3.3. Uncertainties. Several factors influence the estimation of atmospheric emissions, including emission factors and

Table 4. Uncertainties in the Emissions of Hazardous Air Pollutants in China in 2010 HAPs

uncertaintiesa (CI, %)

HAPs

uncertaintiesa (CI, %)

PM (t) NOx (t) SO2 (t) CO (t) HCl (t) PCDD/Fs (t TEQ)

4654.6 (−45.4, 63.4) 28 471.1 (−24.9, 35.3) 12 062.1 (−30.0, 28.2) 6500.5 (−30.7, 46.5) 3609.1 (−48.3, 61.2) 2.4 × 10−5 (−43.3, 41.7)

Sb (t) Hg (t) Pb (t) Cr (t) Ni (t) Cd (t)

69.5 (−10.0, 129.0) 36.7 (−73.8, 100.1) 9.4 (−42.8, 58.9) 4.4 (−88.6, 263.5) 2.8 (−67.6, 144.6) 0.9 (−67.6, 148.2)

As (t)

0.2 (−56.8, 115.8)

a

Expressed as the lower and upper bounds of a 95% confidence interval around a central estimate.

As can be seen from Table 4, hazardous heavy metals are demonstrated to have the highest uncertainties among all studied HAPs. These high uncertainties mainly resulted from poor investigation and inadequate field tests data for Chinese MSW incineration processes. In comparison, PM, gaseous pollutants, and PCDD/Fs show relatively low uncertainties, because these pollutants are the chief criterion pollutants5 in incineration process and there are many field tests to measure the actual emission level of these pollutants from Chinese MSW incineration plants. Thus, more detailed investigation and field tests for all kinds of MSW incineration facilities are still very necessary for a better understanding of the emissions of these HAPs in the future, especially for toxic heavy metals and PCDD/Fs. Although there are still some uncertainties introduced by the lack of original field test data in Chinese MSW incineration plants, this comprehensive emission inventory and temporal and spatial distribution of HAP emissions provide indispensable input data for atmospheric transport, deposition, and abatement strategies for China in future research. 3.4. Possible Countermeasures. Atmospheric emissions of HAPs from MSW incinerators are dependent on the incineration technology and the equipped flue gas purification system as well as the hazardous metals input from the waste. Postcombustion emission control is an effective way to limit the final atmospheric discharge. However, it may not be an optimum solution. Effective HAP emission control can be achieved by limiting the escape of HAPs with the flue gas through air pollution control devices. In addition, refuse sorting and recycling practices can limit and prevent hazardous metals from entering the incinerator. A stringent emission standard and a strong monitoring capacity can ensure reduction in HAP emissions. Effective control of PM, SO2, NOx, CO, HCl, and PCDD/Fs from MSW incineration can be achieved by using advanced combustion technology and effective flue gas cleaning systems. 10369

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will gradually cut the metal content in consumer goods and thus the metal input to wastes. In summary, pollution prevention, advanced combustion technology, and effective flue gas purification systems, as well as public awareness education, are reasonable and useful ways to reduce hazardous air pollutant emission from municipal solid waste incineration and their adverse effects in China.

For example, in order to control PCDD/F emissions, the Chinese authority of MSW management can work from the following aspects: pretreatment of the wastes, emission control of burning process, and postcombustion process. Pretreatment of the waste includes waste sorting and adding combustion improvers (inferior coal, etc.) that can restrain the generation of PCDD/Fs. During the incineration process, improving burning conditions to ensure a stable and full burning environment can reduce the production of precursor, thereby avoiding much synthesis of PCDD/Fs. Activated carbon injection to absorb PCDD/Fs is an effective postcombustion emission control technology.42 The flue gas cleaning system, on the one hand, can reduce the content of PCDD/Fs; on the other hand, it can decrease the concentration of PM and gaseous pollutants.42 Moreover, it is of great importance to establish laws, standards, and regulations that prohibit or restrict backward incineration processes and reliably manage HAPs throughout their life cycles. Additionally, restricting the amount of metals entering incinerators is a significant control measure for preventing hazardous heavy metal emissions from MSW incineration. Thus, the Chinese central and local authorities should actively plan and support programs for collecting and replacing metalcontaining devices from households, such as batteries, thermometers, and fluorescent lamps. In China, a series of measures have been taken to reduce mercury content in batteries and establish relevant recycling programs, whereas little attention has been paid to the disposal of fluorescent lamps (containing up to 10 mg of mercury each).43 In addition to wastes, the mercury content of coal, which is widely used as a supplementary fuel (up to 20%) in MSW incineration in China, should also be effectively reduced (e.g., via coal washing).44 Some in-use MSW incinerators in China were built according to different criteria until the establishment of national standards on pollutant emissions in 2001, which are generally less stringent than those in Europe and the United States.44 A more stringent pollutant emission standard from MSW incineration should be promulgated to halt the increasing trend of air pollutant release from this source category. Existing MSW incineration facilities with poorly designed pollutant control and purification devices should be retrofitted, and advanced effective flue gas purification technologies (best-available control technologies, BACT) for pollutant removal should be required for new facilities. Meanwhile, financial and technical support should be provided to help the industry develop and implement efficient pollutant control technologies. Also, it is very significant to develop an effective administrative mechanism for MSW incineration and a strong monitoring capacity to verify the strict compliance of MSW incineration facilities regarding emissions of HAPs. Finally, compared with pollutant control and reduction after their generation, pollution prevention is a top priority. Reducing and eliminating the metal content in consumer products by the manufacturers is the ultimate solution to metal pollution in MSW management. The Administrative Measure on the Control of Pollution Caused by Electronic Information Products (also known as China RoHS) enacted on March 1, 2007 restricted six groups of hazardous chemicals, including Pb, Hg, Cr, and Cd, in a wide variety of products. Meanwhile, it is significant to develop outreach in communities and support public awareness education programs on the health and environmental risks of heavy metals such as mercury, which are keys to hazardous metal reduction in MSW. These efforts

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ASSOCIATED CONTENT

S Supporting Information *

Three tables listing emission standards, assumed average emission factors, and regional distribution of MSW disposal and incineration capacity in 2010. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; tel: 86-10-58800176; fax: 86-1058800176. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is fund by the National Natural Science Foundation of China (40975061, 21177012). We thank the editors and the anonymous reviewers for their valuable comments and suggestions on our paper.

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REFERENCES

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